Figure 9.5 Snail infection by a pathogen. Shallow water holds more parasites than deep water because ducks defecate and release pathogen eggs more frequently in shallow water. The large number of pathogens allows them to evolve rapidly in response to any changes in the snails. Snails in shallow water are challenged by a need to evolve defenses rapidly against the pathogens that respond quickly. As a result, a much higher percentage of shallow-water snails get infected than deep-water snails. The pathogens also show an increased ability to infect the populations of snails with which they coexist. Pathogens from shallow water are better able to infect shallow-water snails than deep-water snails. Similarly, pathogens from deep water are better able to infect deep-water snails than shallow-water snails.

Data from C. M. Lively and J. Jokela, Clinal variation for local adaptation in a host-parasite interaction, Proceedings of the Royal Society of London B 263 (1996): 891–897.